The invention relates to a method and apparatus for the synchronous mode locking of longitudinal laser modes in a semiconductor diode laser.
Optical light pulses with a duration on the order of picoseconds and fractions thereof may be generated by locking the phases of oscillation modes excited in a laser resonator, i.e. by so-called "mode locking". The minimum pulse width then possible decreases with an increase in the gain band width of the laser-active medium. Consequently laser media with a large gain band width, as for example solutions of organic dyes, F center crystals and semiconductors are more especially suitable for producing pulses of optical radiation of extremely short duration.
A commonly-used form of mode locking is synchronous optical pumping or excitation of a laser by a mode-locked second laser with a lesser band width, see for example the paper of W. H. Glenn et al. in Appl. Phys. Lett. 12, 54, 1968. To cause synchronous mode locking the optical length of the laser producing the pumping pulses has to be equal to the optical length of the laser resonator of the pumped laser or to an integral multiple thereof. Synchronously locked dye laser systems on these lines are commercially available and make it feasible to produce picosecond light pulses in the visible and near infrared range as far as approximately 0.9 .mu.m.
When compared with dye lasers etc. semiconductor lasers may be seen to be characterized by such features as compactness (typical dimensions being 200 .mu.m by 200 .mu.m by 100 .mu.m) and more especially by the simple method of excitation using an electric current and by the small power requirement (typically being some ten to some hundred mW). Furthermore, semiconductor lasers may cover the full spectral range between about 0.7 .mu.m and 30 .mu.m. Semiconductor lasers for the spectral range of 0.7 to 1.6 .mu.m have reached an extraordinarily advanced stage of technical perfection and at room temperature may be operated continuously for periods far in excess of 100,000 hours.
A number of different methods have been developed for mode locking semiconductor lasers, more especially:
(a) Passive mode locking (E. P. Ippen et al., Appl. Phys. Lett. 37, 267 (1980); J. P. van der Ziel et al., Appl. Phys. Lett. 39, 525 (1981)); PA0 (b) Active-passive mode locking (J. P. van der Ziel et al., Appl. Phys. Lett. 39, 867 (1981)); PA0 (c) Active mode locking by gain modulation (J. P. van der Ziel et al., Journal Appl. Phys. 52, 4435 (1981); J. C. AuYeung et al., Appl. Phys. Lett 40, 112 (1982)); PA0 (d) Synchronous mode locking with optical excitation (R. S. Putman et al., Appl. Phys. Lett. 40, 660 (1982)).
For methods (a), (b) and (d) it is not possible to use commercially available semiconductor diode lasers. Method (a) and (b) necessitate an elaborate preparation of one laser end (involving proton or ion bombardment for producing an internal saturable absorber). Generally method (d) requires cooling of the semiconductor laser diode to low temperatures.
In method (c) mode locking is produced by gain modulation, that for its part results from a modulation of the feed current of the semiconductor diode. The modulation of the current takes place purely electronically using radio frequency or pulse generators. As is the case with the methods (a) and (b) as well, in method (c) simultaneous generation of synchronized mode locked radiation pulse trains is not possible at different emission wavelengths. Not one of the above-noted methods is compatible with commercial synchronously pumped dye laser systems.
Furthermore the publication of E. O. Goebel et al. in Appl. Phys. Lett. 42 (1), Jan. 1, 1983, pages 25 to 27 refers to the use of a high-speed optoelectronic GaAs switch to modulate the gain of a semiconductor laser. The switch is controlled by the radiation pulses of a mode locked dye laser. However, the emission of the semiconductor laser is not mode locked.